CREATE RULE

CREATE RULE

Synopsis

Description

CREATE RULE defines a new rule applying to a specified
table or view. CREATE OR REPLACE RULE will either create
a new rule, or replace an existing rule of the same name for the same
table.

The Greenplum Database rule system allows one to define an alternate
action to be performed on insertions, updates, or deletions in database
tables. A rule causes additional or alternate commands to be executed
when a given command on a given table is executed. Rules can be used
on views as well. It is important to realize that a rule is really a
command transformation mechanism, or command macro. The transformation
happens before the execution of the commands starts. It does not operate
independently for each physical row as does a trigger.

ON SELECT rules must be unconditional INSTEAD
rules and must have actions that consist of a single SELECT
command. Thus, an ON SELECT rule effectively turns the
table into a view, whose visible contents are the rows returned by the
rule's SELECT command rather than whatever had been
stored in the table (if anything). It is considered better style to write
a CREATE VIEW command than to create a real table and
define an ON SELECT rule for it.

You can create the illusion of an updatable view by defining ON INSERT,
ON UPDATE, and ON DELETE rules to replace update actions
on the view with appropriate updates on other tables.

There is a catch if you try to use conditional rules for view updates:
there must be an unconditional INSTEAD rule for each
action you wish to allow on the view. If the rule is conditional, or
is not INSTEAD, then the system will still reject attempts
to perform the update action, because it thinks it might end up trying
to perform the action on the dummy table of the view in some cases. If
you want to handle all the useful cases in conditional rules, add an
unconditional DO INSTEAD NOTHING rule to ensure that
the system understands it will never be called on to update the dummy
table. Then make the conditional rules non-INSTEAD;
in the cases where they are applied, they add to the default INSTEAD
NOTHING action. (This method does not currently work to support
RETURNING queries, however.)

Parameters

name

The name of a rule to create. This must be distinct from the name
of any other rule for the same table. Multiple rules on the same table
and same event type are applied in alphabetical name order.

event

The event is one of SELECT, INSERT,
UPDATE, or DELETE.

table

The name (optionally schema-qualified) of the table or view the rule
applies to.

condition

Any SQL conditional expression (returning boolean). The condition
expression may not refer to any tables except NEW and
OLD, and may not contain aggregate functions. NEW
and OLD refer to values in the referenced table. NEW
is valid in ON INSERT and ON UPDATE
rules to refer to the new row being inserted or updated. OLD
is valid in ON UPDATE and ON DELETE
rules to refer to the existing row being updated or deleted.

INSTEAD

INSTEAD indicates that the commands should be executed
instead of the original command.

ALSO

ALSO indicates that the commands should be executed
in addition to the original command. If neither ALSO
nor INSTEAD is specified, ALSO is the
default.

command

The command or commands that make up the rule action. Valid commands
are SELECT, INSERT, UPDATE,
or DELETE. The special table names NEW
and OLD may be used to refer to values in the referenced
table. NEW is valid in ON INSERT and
ONUPDATE rules to refer to the new
row being inserted or updated. OLD is valid in ON
UPDATE and ON DELETE rules to refer to the
existing row being updated or deleted.

Notes

You must be the owner of a table to create or change rules for it.

It is very important to take care to avoid circular rules. Recursive
rules are not validated at rule create time, but will report an error
at execution time.

Examples

Create a rule that inserts rows into the child table b2001 when a user tries to
insert into the partitioned parent table rank:

CREATE RULE b2001 AS ON INSERT TO rank WHERE gender='M' and
year='2001' DO INSTEAD INSERT INTO b2001 VALUES (NEW.id,
NEW.rank, NEW.year, NEW.gender, NEW.count);

Compatibility

CREATE RULE is a Greenplum Database language extension,
as is the entire query rewrite system.